PE loader/image cleanups. No-copy mapping of R2R PEs on Windows. (#61938)

PE loader/image cleanups. No-copy mapping of R2R PEs on Windows. (#61938)

* removed IMAGE_MAPPED

* removed RawImageLayout

* delete MappedImageLayout

* use mem-mapping on Windows

* tweaks and touchups

* a few cleanups

* comments

* move EnsureLoaded into Assemby::Init

* A fix for IsDynamic() case.

* fix for preferred base, if used.

* disable failing scenario

* PR feedback

* Typo (CENTINEL --> SENTINEL)

* added a bug link to a disabled test scenario + couple comment tweaks